Virginia Review News

“We are examining ocean conditions from several past greenhouse climate intervals
so that we can understand better the interactions among the atmosphere, the oceans,
the biosphere, and climate,” said Kenneth MacLeod, professor of geological sciences
in the College of Arts and Science. “The Late Cretaceous Epoch is a textbook example
of a greenhouse climate on earth, and we have evidence that a northern water mass
expanded southwards while the climate was
cooling. At the same time, a warm, salty water mass that had been present throughout
the greenhouse interval disappeared from the tropical Atlantic.”
The study found that at the end of the Late Cretaceous greenhouse interval, water
sinking around Greenland was replaced by surface water flowing north from the South
Atlantic. This change caused the North Atlantic to warm while the rest of the globe
cooled. The change started about five million years before the asteroid impact that
ended the Cretaceous Period.
To track circulation patterns, the researchers focused on “neodymium,” an element
that is taken up by fish teeth and bones when a fish dies and falls to the ocean
floor. MacLeod said the ratio of two isotopes of neodymium acts as a natural tracking
system for water masses. In the area where a water mass forms, the water takes on
a neodymium ratio like that in rocks on nearby land. As the water moves through
the ocean, though, that ratio changes little. Because the fish take up the neodymium
from water at the seafloor, the ratio in the fish fossils reflects the values in
the area where the water sank into the deep ocean. Looking at changes through time
and at many sites allowed the scientists to track water mass movements.
While high atmospheric levels of carbon dioxide caused Late Cretaceous warmth, MacLeod
notes that ocean circulation influenced how that warmth was distributed around the
globe. Further, ocean circulation patterns changed significantly as the climate
warmed and cooled. “Understanding the degree to which climate influences circulation
and vice versa is important today because carbon dioxide levels are rapidly approaching
levels most recently seen during ancient greenhouse times,” said MacLeod. “In just
a few decades, humans are causing changes in the composition of the atmosphere that
are as large as the changes that took millions of years to occur during geological
climate cycles.”
The paper, “Changes in North Atlantic circulation at the end of the Cretaceous greenhouse
interval,” was published in the October online edition of the journal Nature Geoscience.
Coauthors include C. Isaza Londoño of the University of Missouri; E.E. Martin and
C. Basak
of the University of Florida, and A. Jiménez Berrocoso of the Unviersity of Manchester,
United
Kingdom. The study was sponsored by the National Science Foundation.